The overall objective of the proposed research is to increase fundamental understanding of the interaction of high intensity laser radiation with Dental hard tissues and apply that knowledge to the development of laser-based methods for the selective and conservative removal of Dental caries and restorative Dental materials. In addition to eliminating the noise and vibration associated with the Dental drill, lasers offer several unique advantages over current surgical technology for conservative Dentistry. Under the appropriate irradiation conditions, lasers are capable of achieving markedly higher precision and selectivity, inducing beneficial chemical changes in the mineral of the walls of the drilled cavity that render them resistant to further Dental decay, and producing unique surface morphologies that can be exploited to enhance bonding. The Specific Aims of this application are to test the following hypotheses: 1) that a carbon dioxide laser operating at 9.6 mum, resonant with the peak mineral absorption of Dental hard tissues, with a pulse duration commensurate with the thermal relaxation time of the absorbed laser energy, can be used safely and efficaciously for the conservative removal of Dental caries; 2) that the ablation efficiency of Dental enamel at erbium laser wavelengths can be increased significantly by using a pulse duration more commensurate with the thermal and stress relaxation times of the deposited laser energy; 2) that pulsed lasers resonant with strong water, collagen, and hydroxyapatite absorption bands are well suited for the removal of dentin without inducing excessive peripheral thermal damage to the collagen matrix; (4) that the addition of a layer of water to the surface of Dental hard tissue prior to JR laser ablation aids in the removal of secondary non-apatite phases of calcium phosphate through interaction of that irradiated water layer with those loosely adherent phases; (5) that lasers can be used for the selective ablation of resin-based restorative materials without damage to underlying healthy tissues and without excessive heat accumulation. These fundamental studies will significantly advance overall knowledge of laser-tissue interactions and facilitate the development of safer and more efficient laser systems for the removal of Dental caries and restorative materials. Use of those laser systems is likely to lead to the practice of more conservative Dental procedures that will markedly reduce the amount of healthy tissue loss that is generally associated with conventional cavity preparations.